1. Technical Field
The present invention generally relates to a view-finder for photographic camera and more particularly to a bright frame view-finder of natural lighting type therefor.
2. Prior Art
With the conventional bright frame view-finder of natural lighting type, a light image of a field frame illuminated by outdoor daylight or ambient light is introduced into a view-finder optical system at an appropriate point along the optical axis of the view-finder optical system so that the field frame may be superimposed upon an object image to be photographed such that a desired composition of the object image may be determined. FIG. 5 of the attached drawing schematically illustrates an example of such well known bright frame view-finder of natural lighting type. As illustrated, a view-finder optical system comprises an objective 1, an eyepiece 2 and a half-mirror 3 or a half-prism (not shown) interposed between these lenses 1 and 2. There is provided separately of such view-finder optical system an optical system causing the light image of the field frame to appear in the view-finder. Specifically, this light image optical system comprises a light shaft window 4 in the form of a diffusing plate provided on the front surface of camera a and a field frame plate 5 adapted to be illuminated by the outdoor daylight or ambient light passing through said light shaft window 4. Thus, the light images of the field display components such as a field frame 5a and a target mark 5b, which serve for autofocus-ranging and are both defined by transparent portions of the field frame 5 illuminated by the diffused light coming from said light shaft window 4, are reflected substantially at a right angle and then inverted by an intermediate lens 7 to provide the virtual images destined to be superimposed upon an object image to be photographed within the view-finder optical system.
In actual operation of photographing, the field frame plate 5 is illuminated by the outdoor daylight or ambient light passing through the light shaft window 4 in the form of diffusing plate and the images of said field frame 5a and target mark 5b appear in the field of the view-finder optical system while the object to be photographed is observed through the view-finder and the image thereof is caught by the view-finder optical system. With the object caught within the target mark 5b and the desired composition determined by the field frame 5a, a well-focussed picture is obtained.
However, such conventional bright frame view-finder of natural lighting type exclusively relies upon the outdoor daylight or ambient light being incident on the light shaft window 4 in the form of diffusing plate to form the light images of the field display components such as the field frame 5a and the target mark 5b, so it is impossible to cause acceptably clear light images of these field frame 5a and the target mark 5b to appear within the view-finder when the outdoor daylight or ambient light is not sufficiently intense. Consequently, both the determination of desired composition and the ranging by means of autofocussing function could not be achieved in night or indoor photographing.